Transponder systems, together with methods and arrangements for the exchange of data and for the measurement of the distance from a base station to a modulated transponder, exist in diverse forms, and have long been known. General forms of embodiment and principles will be found, for example, in “K. Finkenzeller, RFID-Handbuch, 2 ed. Munich, Vienna: Carl Hanser Verlag, 2000”. Common as transponders are, for example, so-called back-scatter transponders, which do not have their own signal source but simply reflect back the signal they receive, sometimes in amplified form.
In the text which follows such systems, which can be used to measure the distance between the base station and the transponder, are generally described as radio frequency localization systems or RFLO systems (Radio Frequency LOcalisation), by analogy with RFID (Radio Frequency IDentification). Advantageous embodiments of RFLO arrangements of this type, which are based on the principle of the FMCW radar (Frequency Modulated Continuous Wave) or related principles, are described in detail in, for example, “M. Vossiek, R. Roskosch, and P. Heide, Precise 3-D Object Position Tracking using FMCW Radar, 29th European Microwave Conference, Munich, Germany, 1999” and in DE 199 46 161, DE 199 57 536 and DE 199 57 557. DE 199 46 161 indicates methods for the measurement of the separating distance from a transponder, with the classical forms of FMCW backscatter transponders and systems being described here. DE 199 57 536 and DE 199 57 549 describe vehicle access systems, in particular anti-theft systems, embodiments and application, some of which also use FMCW backscatter transponders.
A disadvantage of RFLO backscatter systems of this type is that the signal which is transmitted must propagate across the path from the base station to the transponder and back and accordingly, as determined by the radar equation, the signal-to-noise ratio (SNR) for the entire transmission path falls off in proportion to the 4th power of the distance. The free field attenuation, which increases sharply with the frequency, makes it almost impossible to realize very high frequency passive backscatter transponders in the GHz range, in particular, with a satisfactory signal-to-noise ratio. One particular reason why this is unsatisfactory is because it would in principle be very advantageous to use GHz systems, because of the high available bandwidth, both for distance measurement and also for fast data transmission.
If a separate source is used in the transponder to generate a new signal, based on and phase-coherent with the received signal, then each signal must traverse the base station/transponder path only once. In this case, the signal-to-noise ratio is only inversely proportional to the square of the distance. An additional factor is that the miscellaneous attenuations and losses on the transmission path affect the signal which is transmitted back only once, and not twice. As a result, particularly where the distances are large, the signal-to-noise ratio is larger by orders of magnitude than for simple backscatter systems. However, systems of this type are very much more expensive than the passive backscatter arrangements cited, for example in terms of circuit components, current consumption, manufacturing and maintenance costs, and consequently cannot be considered for many applications.
A further fundamental problem, of RFLO backscatter systems consists, as will be seen from DE 199 46 161 and “M. Vossiek, R. Roskosch, and P. Heide, Precise 3-D Object Position Tracking using FMCW Radar, 29th European Microwave Conference, Munich, Germany, 1999”, in the fact that they are frequently not in a position to measure reliably very short distances between the transponder and the base station. However, it is precisely such short distances which are of particular interest, for example, for access systems and local positioning systems (LPS). The problem is that that it is not possible, mainly for legal but also for technical reasons, to use arbitrarily large modulation bandwidths, B.
As described in the literature sources cited there are, for example with FMCW backscatter RFLO, two spectral components, the distance between which in terms of frequency or phase is proportional to the distance “dist” between the transponder and the base station. The effect of the limited modulation bandwidth is now that the spectral components are not arbitrarily narrow but, when subjected to the usual analysis by a Fourier transformation, have a width of at least Δp=c/(2*B) for physical reasons, where c is the speed of light, B the modulation bandwidth and Ap is the distance here converted to meters.
The physically meaningful frequency of a spectral component corresponds to its maximum value, which normally lies in its center. When the distance is less than a certain minimum, the spectral components overlap. The result of this is that the maximum of the spectral components no longer corresponds to the physically meaningful frequency, and hence there is no magnitude which can be simply read off to determine the frequency, so that the distance can no longer be determined exactly. It is also possible that the two spectral lines extend so far into each other that they are no longer recognizable as separate lines. If the measurement is made with a bandwidth such as 80 MHz, for example, which is the maximum available at 2.45 GHz in the common ISM radar band (ISM=Industrial-Scientific-Medical), which is available worldwide and standardized, then with simple arrangements it is normally no longer possible to detect exact measurement values below a minimum spacing of about 2 m. In practice, indeed, it is normally not possible below 4 m, because in order to calculate the Fourier transformation the time signal is normally weighted by a window function, which has the effect of further degrading the resolution.
According to “M. Vossiek, R. Roskosch, and P. Heide, Precise 3-D Object Position Tracking using FMCW Radar, 29th European Microwave Conference, Munich, Germany, 1999”, this problem can be solved by the use of a delay line, which enforces a fixed basic propagation time for the signal. However, apart from the extra circuitry costs, every dead time in the transponder causes substantial problems in respect of drift and the measurement inaccuracies which result from this.